Resonant switching converter

ABSTRACT

A resonant switching converter can include: a voltage switching circuit configured to receive a first input voltage, and to generate a second input voltage of a resonant branch coupled to the voltage switching circuit; where when the voltage switching circuit is in a first operating state, the second input voltage provided to the resonant branch is less than the first input voltage; and where when the voltage switching circuit is in a second operating state, the input voltage provided to the resonant branch is zero.

RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No. 201811093338.0, filed on Sep. 19, 2018, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to the field of power electronics, and more particularly to resonant switching converters.

BACKGROUND

A switched-mode power supply (SMPS), or a “switching” power supply, can include a power stage circuit and a control circuit. When there is an input voltage, the control circuit can consider internal parameters and external load changes, and may regulate the on/off times of the switch system in the power stage circuit. Switching power supplies have a wide variety of applications in modern electronics. For example, switching power supplies can be used to drive light-emitting diode (LED) loads.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is shown a schematic block diagram of a first example resonant switching converter, in accordance with embodiments of the present invention.

FIG. 2A is shown a schematic block diagram of a second example resonant switching converter, in accordance with embodiments of the present invention.

FIG. 2B is shown a waveform diagram of example operation of the resonant switching converter in FIG. 2A, in accordance with embodiments of the present invention.

FIG. 3A is shown a schematic block diagram of a third example resonant switching converter, in accordance with embodiments of the present invention.

FIG. 3B is shown a waveform diagram of example operation of the resonant switching converter in FIG. 3A, in accordance with embodiments of the present invention.

FIG. 3C is shown a waveform diagram of a fourth example resonant switching converter, in accordance with embodiments of the present invention;

FIG. 4A is shown a schematic block diagram of a fifth example resonant switching converter, in accordance with embodiments of the present invention.

FIG. 4B is shown a waveform diagram of example operation of the resonant switching converter in FIG. 4A in a first control mode, in accordance with embodiments of the present invention.

FIG. 4C is shown a waveform diagram of example operation of the resonant switching converter in FIG. 4A in a second control mode, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

Reference may now be made in detail to particular embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention may be described in conjunction with the preferred embodiments, it may be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents that may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it may be readily apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, processes, components, structures, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention.

With the development of power electronics, switching converters have become more and more widely used. Correspondingly, the design of switching converters is also facing more and more challenges, including comprehensive performance requirements of high power density, high efficiency, fast speed and dynamic characteristics, and so on. In order to achieve higher power density, the approach of increasing the switching frequency is generally utilized to reduce the capacitance and the volume of magnetic components, such as transformers and inductors, but the increase of the switching frequency leads to an increase in switching loss, thereby reducing the efficiency of the switching converter.

Soft switching technology is one approach for the switching converter to achieve high frequency operation. Using the principle of resonance, the current or voltage of the power devices in the switching converter varies with the quasi-sinusoidal law, thereby realizing soft switching, and solving potential problems of hard switching loss in the hard switching converter, capacitive conduction, inductive shutdown, reverse diode recovery, and electromagnetic interference (EMI), that may be caused by hard switching. Isolated converters (e.g., flyback converters, forward converters, half-bridge converters, etc.) can realize soft switching by use of quasi-resonant, but potential drawbacks of this approach can include large stresses of the power devices, large power losses, low power density caused by limitation of the switching frequency, and large turns ratio and volume of the transformer.

In one embodiment, a resonant switching converter can include: (i) a voltage switching circuit configured to receive a first input voltage, and to generate a second input voltage of a resonant branch coupled to the voltage switching circuit; (ii) where when the voltage switching circuit is in a first operating state, the second input voltage provided to the resonant branch is less than the first input voltage; and (iii) where when the voltage switching circuit is in a second operating state, the input voltage provided to the resonant branch is zero.

Referring now to FIG. 1, shown is a schematic block diagram of a first example resonant switching converter, in accordance with embodiments of the present invention. As shown in FIG. 1A, resonant switching converter 100 can include voltage switching circuit 11, resonant branch 12 and output circuit 13. Voltage switching circuit 11 can receive input voltage V_(in), and may generate input voltage V_(r) having at least two different values. Resonant branch 12 can be coupled to voltage switching circuit 11, and may receive input voltage V_(r). Voltage switching circuit 11 may have at least two operating states during each switching cycle. In a first operating state, voltage switching circuit 11 can control input voltage V_(r) of resonant branch 12 to be not greater than input voltage V_(in). In a second operating state, voltage switching circuit 11 can control input voltage V_(r) of resonant branch 12 to be zero.

In this example, voltage switching circuit 11 can include at least two power transistors. A resonant tank of the resonant switching converter can be formed with resonant branch 12 and at least one power transistor in voltage switching circuit 11, in order to achieve resonant control. Output circuit 13 may be coupled to resonant branch 12 to receive the energy transferred by the resonant tank, and may accordingly generate an output electrical signal to drive a load. In this particular example, a flyback converter topology is employed. Resonant branch 12 can include resonant capacitor C_(r), resonant inductor L_(r), magnetize inductance L_(m), and primary winding n_(p) of transformer T connected in series between two terminals of input voltage V_(r). Output circuit 13 can include series-connected secondary winding ns of transformer T, freewheeling diode Do and output capacitor Co. It should be understood that the actual transformer can be equivalent to a combination of leakage inductance, magnetize inductance and ideal transformer. In some applications, the leakage inductance can be used as a resonant inductance. As another example, resonant inductor L_(r) may be the leakage inductance of the transformer, or it may be a separate inductor or a combination of a separate inductor and the leakage inductance of the transformer.

During the operating process, the value of input voltage V_(r) can vary alternately according to application requirements, and may be maintained at a value not greater than input voltage V_(in). In switching cycle T, when the value of input voltage V_(r) is not zero and not greater than input voltage V_(in), magnetic inductor current i_(m) and primary winding current i_(p) can increase linearly, and the values of both can be equal. Further, no energy may transmitted to output circuit 13, and freewheeling diode Do may have no current flowing therethrough. When the value of input voltage V_(r) is switched to zero, magnetic inductor current i_(m) may decrease linearly, primary winding current i_(p) may be a resonant current, and the difference between magnetic inductor current i_(m) and primary winding current i_(p) can be transmitted to output circuit 13 though secondary winding n_(s), such that current io may flow through freewheeling diode Do. Repeatedly, as the value of input voltage V_(r) is switched, the values of magnetic inductor current i_(m) and primary winding current i_(p) can be cyclically switched, and output circuit 13 may generate a corresponding output electrical signal.

In particular embodiments, the resonant branch can multiplex at least one power transistor in the voltage switching circuit as a main power switch or an auxiliary power switch, which can form the resonant tank with the resonant branch including resonant elements. Moreover, the voltage stress of the power transistor can be reduced by the voltage switching circuit for converting the input voltage of the resonant switching converter while realizing the resonance control, such that the transistor with lower performance parameter can be utilized. In addition, switching loss, and performance requirements of other devices in the circuit can be correspondingly reduced (e.g., turns ratio of the transformer, etc.), thereby reducing the size and cost of the system circuit. On the other hand, the multiple resonant tanks can transfer energy to the output circuit of the resonant switching converter. For example, at least one resonant tank can transmit energy to the output circuit at any time during the operating process, such that the ripple of the output circuit may be reduced, and the output voltage may be more stable.

In this example, voltage switching circuit 11 in the resonant switching converter can include: N half-bridge circuits coupled in series between two terminals of input voltage V_(in), each of which can include two power transistors connected in series, (N-1) first capacitors connected between output terminals of the two adjacent half-bridge circuits, and (N-1) second capacitors connected between two input terminals of a corresponding one of the second to the Nth half-bridge circuits respectively, where N≥2. Resonant branch 12 can connect between two power terminals of the 2Nth power transistor. The input voltage of resonant branch 12 can be switched by controlling the switching state of the power transistors and the voltage division of the first and second capacitors. In addition, resonant branch 12 can multiplex the power transistors in voltage switching circuit 11, in order to achieve resonant control.

Referring now to FIG. 2A, shown is a schematic block diagram of a second example resonant switching converter, in accordance with an embodiment of the present invention. In this example, voltage switching circuit 11 can include a first half-bridge circuit including series-connected power transistors Q1 and Q2, and a second half-bridge circuit including series-connected power transistors second Q3 and Q4. The first and second half-bridge circuits can connect in series between two terminals of input voltage V_(in). Resonant branch 12 can connect between two power terminals of power transistor Q4. Further, voltage switching circuit 11 can include capacitor C_(in1) connected between output terminal A of the first half-bridge circuit and output terminal B of the second half-bridge circuit, and capacitor C_(in2) connected between input terminals of the second half-bridge circuit.

Referring now to FIG. 2B, shown is a waveform diagram of example operation of the resonant switching circuit in FIG. 2A, in accordance with embodiments of the present invention. In each switching period T, the switching states of transistors Q1 and Q3 can be the same, the switching states of transistors Q2 and Q4 may be the same, and the switching states of transistors Q1 and Q2 can be complementary. In this particular example, in switching period T, voltage switching circuit 11 may have two operating states, namely time intervals [t₀-t₁] and [t₁-t₂], respectively. During time interval to-ti, switching circuit 11 can operate in the first operating state, power transistors Q1 and Q3 may be turned on (e.g., at the same time), and power transistors Q2 and Q4 can be turned off, such that conduction paths are Vin-Q1-Cin1-Cr-Lr-Lm-ground terminal, and Cin2-Q3-Cr-Lm-ground terminal. Further, capacitor C_(in1) can be in a charging state, capacitor C_(in2) may be in a discharging state, and the input voltages of resonant branch 12 can be voltage Vin/2 across capacitor C_(in2). In this way, magnetic inductor current i_(m) and primary winding current i_(p) may both increase linearly, and the values of both may be equal, such that transformer T can store energy, no energy may be transmitted to output circuit 13, and no current may flow through freewheeling diode Do.

During time interval t₁-t₂, switching circuit 11 can operate in the second operating state, power transistors Q2 and Q4 can be turned on (e.g., at the same time), and power transistors Q1 and Q3 may be turned off, such that capacitors C_(in1) and C_(in2) can effectively connect in parallel, the voltages across capacitors C_(in1) and C_(in2) are equal. Therefore, capacitors C_(in1) and C_(in2) may not need to be matched to have the same capacitance value; that is, the capacitance values of the two can be the same or different. The conduction path may be Q4-Cr-Lr-Lm (n_(p))-ground terminal during time interval t₁-t₂, and the input voltage (e.g., input voltage V_(r)) of resonant branch 12 can be zero. In this way, magnetic inductor current i_(m) can decrease linearly, and primary winding current i_(p) may be the resonant current, such that the difference between magnetic inductor current i_(m) and primary winding current i_(p) can be transmitted to output circuit 13 through secondary winding n_(s), energy can be transmitted to output circuit 13, and a current may flow through freewheeling diode Do. Repeatedly, as the value of voltage signal V_(r) may be switched, the values of magnetic inductor current i_(m) and primary winding current i_(p) may be cyclically switched, and output circuit 13 can generate the corresponding output electrical signal.

In this example, a part of the power transistors in the voltage switching circuit can be multiplexed, and may form the resonance tank with resonance branch 12, in order to realize resonance control. In the first operating state, power transistors Q1 and Q3 can be multiplexed as the main power switch, and in the second operating state, power transistor Q4 can be multiplexed as the auxiliary power switch, such that the resonant tank may be equivalent to an asymmetric flyback converter for achieving resonance control, and zero voltage switching (ZVS) of the power transistors. Moreover, the voltage stress of the power transistor can be reduced to half of the input voltage Vin/2 by controlling the switching states of the four power transistors and the voltage division of capacitors C_(in1) and C_(in2) while realizing the resonance control, such that the transistor with lower performance parameter(s) can be utilized, and performance requirement(s) of other devices in the circuit can be correspondingly reduced (e.g., turns ratio of the transformer, etc.), thereby reducing the size and cost of the system circuit.

Referring now to FIG. 3A, shown is a schematic block diagram of a third example resonant switching converter, in accordance with embodiments of the present invention. In this particular example, voltage switching circuit 11 can include a first half-bridge circuit including series-connected power transistors Q1 and Q2, and a second half-bridge circuit including series-connected power transistors Q3 and Q4. The first and second half-bridge circuits can connect in series between two terminals of input voltage V_(in). Resonant branch 12 can connect between two power terminals of power transistor Q4. Further, voltage switching circuit 11 can include capacitor C_(in1) connected between output terminal A of the first half-bridge circuit and output terminal B of the second half-bridge circuit, capacitor C_(in2) connected between input terminals of the second half-bridge circuit, and capacitor C_(in3) connected between input terminals of the first half-bridge circuit. In each switching period T, the switching states of transistors Q1 and Q3 may be the same, the switching states of transistors Q2 and Q4 may be the same, and the switching states of transistors Q1 and Q2 can be complementary.

Referring now to FIG. 3B, shown is a waveform diagram of example operation of the resonant switching circuit in FIG. 3A, in accordance with embodiments of the present invention. During switching period T, voltage switching circuit 11 may have two operating states, time intervals [t₁-t₁] and [t₁-t₂]. During time interval t₀-t₁, voltage switching circuit 11 can operate in a first operating state, transistors Q1 and Q3 may be turned on, and transistors Q2 and Q4 can be turned off, such that a first conduction path can be Vin-Q1-Cin1-Cr-Lr-Lm-ground terminal, a second conduction path may be Vin-Cin3-Q3-Cr-Lr-Lm-Ground terminal, and a third conduction path can be Cin2-Q3-Cr-Lr-Lm-ground terminal. Further, capacitors C_(in1) and C_(in3) can effectively connect in parallel between the positive terminal of input voltage V_(in) and common node B of power transistors Q3 and Q4, and the voltages across capacitors C_(in1) and C_(in3) may be equal. During this time interval, the input voltage of resonant branch 12 may be voltage Vin/2 across capacitor C_(in2). In this way, magnetic inductor current i_(m) and primary winding current i_(p) may both increase linearly, and the values of both can be equal, such that transformer T can store energy, no energy may be transmitted to output circuit 13, and no current may flow through freewheeling diode Do.

During time interval t₁-t₂, voltage switching circuit 11 can operate in a second operating state, transistors Q1 and Q3 can be turned off, and transistors Q2 and Q4 may be turned on, such that the conduction path may be Q4-Cr-Lr-Lm-ground terminal. Further, capacitors C_(in1) and C_(in2) can effectively connect in parallel between the negative terminal of input voltage V_(in) and common node A of power transistors Q1 and Q2, and the voltages across capacitors C_(in1) and C_(in2) can be equal. During this time interval, the input voltage of resonant branch 12 can be zero. In this way, magnetic inductor current i_(m) can decrease linearly, and primary winding current i_(p) may be the resonant current, such that the difference between magnetic inductor current i_(m) and primary winding current i_(p) can be transmitted to output circuit 13 through secondary winding n_(s), energy can be transmitted to output circuit 13, and a current may flow through freewheeling diode Do. Repeatedly, as the value of input voltage V_(r) may be switched, the values of magnetic inductor current i_(m) and primary winding current i_(p) can be cyclically switched, and output circuit 13 can generate the corresponding output electrical signal.

In this example, the operating states of the power transistors can be controlled in the above two time intervals, capacitors C_(in1), C_(in2), and C_(in3) may not need to be matched to have the same capacitance value, and the capacitance values of the three capacitors can be the same or at least two may not be the same. In this example, a part of the power transistors in the voltage switching circuit can be multiplexed, and may form the resonance tank with resonance branch 12, in order to realize resonance control. In the first operating state, power transistors Q1 and Q3 can be multiplexed as the main power switch, and in the second operating state, power transistor Q4 can be multiplexed as the auxiliary power switch, such that the resonant tank may be equivalent to a asymmetric flyback converter for achieving resonance control, and ZVS of the power transistors can also be achieved. Moreover, the voltage drop of the power transistors can be reduced to half of the input voltage Vin/2 by controlling the switching state of the four power transistors and the voltage division of capacitors C_(in1), C_(in2), and C_(in3), such that the transistor with lower performance parameter(s) can be utilized, and performance requirement(s) of other devices in the circuit can be correspondingly reduced (e.g., turns ratio of the transformer, etc.), thereby reducing the size and cost of the system circuit.

Referring now to FIG. 3C, shown is a schematic block diagram of a fourth example resonant switching converter, in accordance with an embodiment of the present invention. In this particular example, the input terminals of resonant branch 12 can be switched between a positive terminal of input voltage V_(in) and the voltage at the common node between power transistors Q1 and Q2, as compared with the resonant switching converter shown in FIG. 3A.

In the above examples, energy can be transferred to the output circuit only in the second operating state, and in some cases the output ripple may be relatively large. Therefore, the resonant switching converter in another example can include N output branches having power transistors for receiving input voltage V_(in) and generating N “second” input voltages, as well as a first capacitor connected between output branches. In this example, each of the output branches can be coupled to a corresponding one of the resonant branches, and each of the resonant branches can be coupled between two power terminals of one power transistor in a corresponding one of the output branches, where N≥2. In the first operating state, the second input voltage may be supplied to the resonant branches through input voltage V_(in) and the first capacitor, or only through the first capacitor. In this example, the multiple resonant branches can transfer energy to the output circuit, such that the output ripple can be reduced. For example, at least one resonant branch can transfer energy to the output circuit of the resonant switching converter at each moment, such that the energy of the output circuit can be continuous, thereby improving the stability of the output electrical signal.

Referring now to FIG. 4A, shown is a schematic block diagram of a fifth example resonant switching converter, in accordance with an embodiment of the present invention. This particular example resonant switching converter can include two resonant branches. Voltage switching circuit 520 of the resonant type switching converter can include two output branches, and may generate the second input voltages to the resonant branches, the number of which can be consistent with the number of resonant branches. For example, each second input voltage may have at least two values, and the value of each second input voltage may not be greater than input voltage V_(in). In this example, voltage switching circuit 520 can include power transistors Q1, Q2, Q3, and Q4 and capacitor C_(in), where the first output branch can include power transistors Q1, Q2, and Q3 connected in series between the two terminals of input voltage V_(in), the second output branch can include power transistors Q1 and Q4 coupled in series between two terminals of input voltage V_(in), and capacitor C_(in) can connect between power transistors Q1 and Q4. Each resonant branch can connect between two power terminals of a corresponding one of the power transistors. For example, resonant branch 521 can connect between the drain and the source of power transistor Q3 in the first output branch, and resonant branch 522 can connect between the drain and the source of power transistor Q4 in the second output branch.

During each switching cycle, the switching states of power transistor Q1 and Q4 may be complementary, and the switching states of power transistor Q2 and Q3 can be complementary. Different operating states can be obtained by controlling the phase difference between power transistor Q2 and Q1 to be 180°, and adjusting duty cycles of power transistors Q1 and Q2. As another example, different operating states may be obtained by adjusting the phase difference between power transistors Q2 and Q1. When in a second operating state, power transistor Q3 or Q4 can be turned on, and the input voltage of the corresponding resonant branch may be switched to zero. When in a first operating state, power transistor Q3 or power transistor Q4 can be turned off, power transistor Q1 or Q2 may be turned on, the input voltage of the corresponding resonant branch can be provided through input voltage V_(in) and capacitor C_(in), or only through capacitor C_(in).

Referring now to FIG. 4B, shown is a waveform diagram of example operation of the resonant switching converter in FIG. 4A in a first control mode, in accordance with embodiments of the present invention. In the first control mode, the resonant switching converter may have four states during each switching cycle T, namely time intervals [t₀-t₁], [t₁-t₂], [t₂-t₃], and [t₃-t₄] respectively. During time interval t₀-t₁, power transistors Q1 and Q3 can be turned on at the same time, and power transistors Q2 and Q4 may be turned off. The first conduction path can be Q3-Cr1-Lr1-Lm1-ground terminal, and “second” input voltage V_(r1) of resonance branch 521 may be zero. The second conduction path can be Vin-Q1-Cin-Cr2-Lr2-Lm2-ground terminal, and input voltage V_(r2) of resonant branch 522 may be the difference between input voltage V_(in) and the voltage across capacitor C_(in); that is, Vin/2. In this way, resonant branch 521 can transmit power to output circuit 523 through transformer T1. Further, magnetic inductor current i_(m2) and primary winding current i_(p2) may both increase linearly, and the values of both can be equal, such that transformer T2 can store energy, and no energy may be transmitted to output circuit 523 through transformer T2.

During time interval t₁-t₂, power transistors Q3 and Q4 can be turned on at the same time, and power transistors Q1 and Q2 are turned off. The first conduction path may be Q3-Cr1-Lr1-Lm1-ground terminal, and input voltage V_(r1) of resonant branch 521 can be zero. The second conduction path may be Q4-Cr2-Lr2-Lm2-ground terminal, and input voltage V_(r2) of resonant branch 522 can be zero. In this way, resonant branch 521 can transmit power to output circuit 523 through transformer T1 and a current may flow through freewheeling diode Do1 according to the resonant current of the resonant tank and the resonant cycle. In this operating state, resonant branch 522 can transmit power to output circuit 523 through transformer T2.

During time interval t₂-t₃, power transistors Q2 and Q4 may be turned on at the same time, and power transistors Q1 and Q3 can be turned off. The first conduction path may be Q4-Cin-Q2-Cr1-Lr1-Lm1-ground terminal, and input voltage V_(r1) of resonant branch 521 can be the voltage across capacitor C_(in); that is, Vin/2. The second conduction path may be Q4-Cr2-Lr2-Lm2-ground terminal, and input voltage V_(r2) of resonant branch 522 can be zero. In this way, magnetic inductor current i_(m1) and primary winding current i_(p1) may both increase linearly, and the values of both may be equal, such that transformer T1 can store energy, and no energy may be transmitted to output circuit 523 through transformer T1. In this operating state, resonant branch 522 can transmit power to output circuit 523 through transformer T2 and a current may flow through freewheeling diode Do1 according to the resonant current of the resonant tank and the resonant cycle.

During time interval t₃-t₄, power transistors Q3 and Q4 can be turned on (e.g., at the same time), and power transistors Q1 and Q2 may be turned off. The first conduction path can be Q3-Cr1-Lr1-Lm1-ground terminal, and input voltage V_(r1) of resonant branch 521 may be zero. The second conduction path can be Q4-Cr2-Lr2-Lm2-ground terminal, and input voltage V_(r2) of resonant branch 522 may be zero. In this way, resonant branch 521 can transmit power to output circuit 523 through transformer T1 according to the resonant current of the resonant tank and the resonant cycle. Further, resonant branch 522 can transmit power to output circuit 523 through transformer T1 according to the resonant current of the resonant tank and the resonant cycle.

Referring now to FIG. 4C, shown is a waveform diagram of example operation of the resonant switching converter in FIG. 4A in a second control mode, in accordance with embodiments of the present invention. In the second control mode, the resonant switching converter may have two different states during each switching cycle T, namely time intervals [t₀-t₁], [t₁-t₂], [t₂-t₃], and [t₃-t₄]. During each switching cycle, the switching states of power transistors Q1 and Q4 can be complementary, the switching states of power transistors Q2 and Q3 may be complementary, and the switching states of power transistors Q1 and Q3 can be the same.

During time interval t₀-t₁, power transistors Q1 and Q3 may be turned on (e.g., at the same time), and power transistors Q2 and Q4 are turned off. The first conduction path can be Q3-Cr1-Lr1-Lm1-ground terminal, and input voltage V_(r1) of resonance branch 521 may be zero. The second conduction path can be Vin-Q1-Cin-Cr2-Lr2-Lm2-ground terminal, and input voltage V_(r2) of resonant branch 522 may be the difference between input voltage V_(in) and the voltage across capacitor C_(in); that is, Vin/2. In this way, resonant branch 521 can transmit power to output circuit 523 through transformer T1. Further, magnetic inductor current i_(m2) and primary winding current i_(p2) may both increase linearly, and the values of both may be equal, such that transformer T2 can store energy, and no energy may be transmitted to output circuit 523 through transformer T2.

During time interval t₁-t₂, power transistors Q2 and Q4 can be turned on at the same time, and power transistors Q1 and Q3 may be turned off. The first conduction path can be Q4-Cin-Q2-Cr1-Lr1-Lm1-ground terminal, and input voltage V_(r1) of resonant branch 521 may be the voltage across capacitor C_(in); that is, Vin/2. The second conduction path can be Q4-Cr2-Lr2-Lm2-ground terminal, and input voltage V_(r2) of resonant branch 522 may be zero. In this way, magnetic inductor current i_(m1) and primary winding current i_(p1) may both increase linearly, and the values of both may be equal, such that transformer T1 can store energy, and no energy may be transmitted to output circuit 523 through transformer T1. In this operating state, resonant branch 522 can transmit power to output circuit 523 through transformer T2 according to the resonant current of the resonant tank and the resonant cycle, and a current may flow through freewheeling diode Do2.

In the second operating mode, when power transistor Q3 may be turned on, the input voltage of resonant branch 521 can be switched to zero, and when power transistor Q4 is turned on, the input voltage of resonant branch 522 can be switched to zero. In the first operating mode, when power transistor Q3 or Q4 is turned off, power transistor Q1 or Q2 can be turned on, such that the input voltage of resonant branch 521 may be provided through capacitor C_(in), and the input voltage of resonant branch 522 can be provided through input voltage V_(in) and capacitor C_(in).

In this example, the number of the second input voltages (e.g., V_(r1), V_(r2), etc.) generated by the voltage switching circuit may be consistent with the number of resonant branches, in order to provide the input voltages to the resonant branches. The multiple resonant branches can transfer energy to the output circuit, such that output ripple can be reduced. For example, at least one resonant branch can transfer energy to the output circuit of the resonant switching converter at each moment, such that the energy of the output circuit can be continuous, thereby improving the stability of the output electrical signal.

In particular embodiments, a part of the power transistors in the voltage switching circuit can be multiplexed, and may form the resonance tank with the resonance branch, in order to realize resonance control. Moreover, the voltage drop of the power transistors can be reduced, such that the transistors with lower performance parameter can be utilized, and performance requirement(s) of other devices in the circuit can be correspondingly reduced (e.g., turns ratio of the transformer, etc.), thereby reducing the he size and cost of the system circuit.

For example, the isolated flyback converter topology can be employed, the circuit structure of the voltage switching circuit can be a four-switch voltage conversion structure, and the voltage division ratio of the input voltage may be 1/2. However, other types of topologies can be applied, such as an asymmetric half-bridge forward converter, and other structures of the voltage switching circuit, such as a voltage conversion structure with six or eight switches, and voltage divider capacitors or other types of suitable voltage conversion structures, can also be employed in certain embodiments.

The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with modifications as are suited to particular use(s) contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents. 

1.-20. (canceled)
 21. A resonant switching converter, comprising: a) a voltage switching circuit configured to receive a first input voltage, and to generate a second input voltage that is correspondingly provided to a resonant branch coupled to said voltage switching circuit; b) wherein said voltage switching circuit comprises a plurality of power transistors coupled in series between an input terminal and a ground terminal of said voltage switching circuit; c) wherein when said voltage switching circuit is in a first operating state, said second input voltage provided to said resonant branch is less than said first input voltage; and d) wherein when said voltage switching circuit is in a second operating state, said second input voltage correspondingly provided to said resonant branch is zero.
 22. The resonant switching converter of claim 21, wherein said resonant branch is coupled between two power terminals of a corresponding one of said plurality of power transistors.
 23. The resonant switching converter of claim 21, wherein a voltage at a first input terminal of said resonant branch is constant in each switching period and a voltage at a second input terminal of said resonant branch is changed according to switching states of said plurality of power transistors.
 24. The resonant switching converter of claim 21, wherein a resonant tank of said resonant switching converter is formed with at least one power transistor of said voltage switching circuit and said resonant branch.
 25. The resonant switching converter of claim 21, wherein said first and second operating states alternate during each switching cycle.
 26. The resonant switching converter of claim 21, wherein said voltage switching circuit comprises: a) N half-bridge circuits coupled in series between two terminals of said first input voltage, wherein each half-bridge circuit comprises two power transistors coupled in series, and wherein N is a positive integer of at least two; b) N-1 first capacitors, each of which being coupled between output terminals of two adjacent half-bridge circuits; and c) N-1 second capacitors respectively coupled between two input terminals of a corresponding one of a second to a N-th half-bridge circuits.
 27. The resonant switching converter of claim 26, wherein said resonant branch is coupled between two power terminals of a 2N-th power transistor of said N-th half-bridge circuit.
 28. The resonant switching converter of claim 26, wherein said resonant branch is coupled between two power terminals of a first power transistor of said first half-bridge circuit.
 29. The resonant switching converter of claim 26, wherein: a) when in said first operating state, said resonant branch multiplexes N power transistors in said N half-bridge current units as a main power transistor; and b) when in said second operating state, said resonant branch multiplexes said power transistor that is coupled between said input terminals of said resonant branch as an auxiliary power transistor, in order to form said resonant switching converter.
 30. The resonant switching converter of claim 21, wherein said voltage switching circuit comprises 2N power transistors, and said second input voltage is consistent with 1/N of said first input voltage.
 31. The resonant switching converter of claim 26, wherein when in said first operating state, power transistors with odd numbers in 2N series power transistors are in on states, and power transistor with even numbers in 2N series power transistors are in off states.
 32. The resonant switching converter of claim 21, wherein said voltage switching circuit comprises: a) N half-bridge circuits coupled in series between two terminals of said first input voltage, wherein each half-bridge circuit comprises two power transistors coupled in series, and, wherein N is a positive integer of at least two; b) N-1 first capacitors, each of which being coupled between output terminals of two adjacent half-bridge circuits; and c) N second capacitors respectively coupled between two input terminals of a corresponding one of half-bridge circuits.
 33. The resonant switching converter of claim 32, wherein said resonant branch is coupled between two power terminals of a 2N-th power transistor of said N-th half-bridge circuit.
 34. The resonant switching converter of claim 32, wherein said resonant branch is coupled between two power terminals of a first power transistor of said first half-bridge circuit.
 35. The resonant switching converter of claim 32, wherein in said first operating mode, power transistors with odd numbers in 2N series power transistors are in on states, and power transistor with even numbers in 2N series power transistors are in off states.
 36. The resonant switching converter of claim 22, wherein said resonant switching converter comprises N resonant branches, said second input voltages provided by said voltage switching circuit to said resonant branches are 1/N of said first input voltage.
 37. The resonant switching converter of claim 36, wherein power is transmitted to an output circuit of said resonant switching converter through at least one resonant branch at each time.
 38. The resonant switching converter of claim 36, wherein: a) each said resonant branch is coupled between two power terminals of a corresponding one of power transistors having one power terminal coupled to said ground terminal in said voltage switching circuit; and b) at least one said corresponding one of power transistors is turned on at said second operating state.
 39. The resonant switching converter of claim 36, wherein by controlling switching states of said power transistors, said second input voltage is provided to said resonant branch through said first input voltage and said first capacitor, or only through said first capacitor.
 40. The resonant switching converter of claim 36, wherein said voltage switching circuit comprises: a) a first output branch having first, second, and third power transistors coupled in series between two terminals of said first input voltage; b) a second output branch having said first power transistor, a first capacitor, and a fourth power transistor coupled in series between two terminals of said first input voltage; and c) wherein a first resonant branch is coupled between two power terminals of said third power transistor, and a second resonant branch is coupled between two power terminals of said fourth power transistor.
 41. The resonant switching converter of claim 40, wherein switching states of said first and fourth power transistors are complementary, and switching states of said second and third power transistors are the same.
 42. The resonant switching converter of claim 40, wherein power transmitted to said output circuit of said resonant switching converter by said N resonant branches is regulated by adjusting phase difference between said first and second power transistors, or by adjusting duty cycles of said first and second power transistors.
 43. The resonant switching converter of claim 40, wherein said voltage switching circuit comprises N output branches, each of said output branch is coupled between said first input voltage and a ground, and being configured to provide said second input voltage to a corresponding one of said N resonant branches. 